1. Field of the Invention
The present invention generally relates to wireless data communication; and in particular, the present invention relates to network access protocols used in wireless data communication.
2. Background of the Invention
An example of a network access protocol for a two-way wireless data network is shown in FIG. 1 and described in detail in commonly assigned U.S. Patent Application ("'860 Application"), Ser. No. 08/542,860, entitled "TWO-WAY WIRELESS DATA NETWORK", by Weijia Wang, now U.S. Pat. No. 5,898,904, issued on Apr. 27, 1999. The '760 Application is incorporated herein by reference in its entirety.
As shown in FIG. 1, a wireless data network 100 includes a wireless data terminal 101, a cellularized base station 102, a message control center 103, interfaces 105-109 to information and communication applications, and radio links 115 and 117. Message control center 103 provides a high power transmitter capable of broadcasting over a paging channel to wireless data terminals within the entire service area of wireless data network 100. In this manner, wireless data network 100 is compatible with existing one-way paging services. Message control center 103 also communicates with base station 102 through radio link 117 which is a high power transmission (e.g. 3 watts).
Base station 102 is one of a number of base stations that are distributed throughout the service area of wireless data network 100. Each base station serves a portion of the service area of wireless data network 100 within its immediate vicinity. The local service area of a base station is sometimes called a "cell". The base stations broadcast to wireless data terminals in their respective cells through a local channel. Typically, the base stations cumulatively serve all locations within the service area of the wireless data network 100. Wireless data terminal 101 communicates with one or more of the base stations, e.g., base station 102, through radio link 115. Radio link 115 needs only provide a low power transmission (e.g. 100 mW) to service the local service area. When wireless data terminal is outside the local service area of any base station, wireless data terminal 101 is restricted only to receiving messages from the 1-way paging channel. Other details of the operation of the two-way wireless data network can be found in the aforementioned '860 Application and incorporated by reference above.
Enhancements to the network access scheme described above have been suggested so as to increase the network bandwidth and channel utilization of a two-way wireless data network. One such enhancement is described the Copending Application incorporated by reference above. The Copending Application discloses a network access protocol, referred to as the capture division packet access (CDPA) protocol, which allows a wireless data terminal to camp onto a local base station for two-way communication whenever the wireless data terminal is within the service range of the base station, thus bypassing the message control center all together. Under the "local override" mode of the CDPA protocol, a wireless data terminal registers with a local base station upon entering the local service area of the local base station, and subsequently communicates with that base station over the local channel. The local override mode avoids the undesired latency associated with high message traffic conditions in the paging channel. The CDPA protocol also supports a "local-only" mode in which the wireless data terminal ignores communication over the paging channel and camps onto the local channel at all times for two-way communication.
The CDPA protocol described in the Copending Application also includes other performance enhancement features. For instance, the CDPA protocol supports asymmetric downlink and uplink coverage areas in the local channel. The coverage pattern, i.e., the downlink coverage area is larger than the uplink coverage area, allows incremental start-up of a new 2-2-way wireless communication system. Under an asymmetric coverage area scheme, a wireless data terminal carries out two-way communication with a base station while within the uplink coverage area, and receives messages from the base station when outside the uplink coverage area but still within the downlink coverage area.
The CDPA protocol described in the Copending Application uses a variable speed retry scheme to avoid collision in the uplink and to extend the range of uplink communication. Under the variable speed retry scheme, when a previous transmission is unsuccessful, a wireless data terminal retransmits the data packet using alternatively a high data rate transmission and a low data rate transmission. Furthermore, the retransmissions are attempted at randomized time intervals so as to minimize collision with other wireless data terminals competing for the uplink. Other aspects of the CDPA protocol are described in details in the Copending Application incorporated by reference above.
The CDPA protocol described above is particularly useful when a wireless data terminal communicates primarily with a small number of local base stations. As described in the Copending Application, an example of such an application is found in a hospital where a user typically receives messages originated and received within the hospital during the normal work day. In that application, the hospital usually can be serviced by at most a small number of base stations under the control of a local message control center, forming what is sometimes referred to as a "private" system. The CDPA protocol supports wireless network communication both within the private system and outside the private system, sometimes referred to as the "public" system.
Performance problems can arise in the application described above where two-way communication is provided by a private system operating within a public system. For example, to provide coverage within a private system, base stations are sometimes installed closer together than they need to be in the counterpart public system. Such close deployment of base stations can result in overlapping coverage areas in the open space within the service area of the private system. When a wireless data terminal is operating in a coverage area of overlap, the wireless data terminal receives communications from more than one base station. However, interference among these base stations ("co-channel interference") can occur, causing the wireless data terminal to receive no packet at all or unable to transmit to any of the base stations. A network access scheme capable of minimizing co-channel interference improves efficiency of the two-way wireless data network.
In the two-way wireless data network described above, wireless data terminals compete for the uplink bandwidth. Collisions resulting from simultaneous transmissions by two or more wireless data terminals can occur to significantly reduce the channel utilization rate. For instance, packet collisions are very common under the ALOHA protocol because wireless data terminals transmit without regard to channel availability. Collision avoidance schemes, such as the variable speed retry scheme described above, increase channel utilization.